Our invention relates to an apparatus for data transfer with interchangeable disklike record media such as flexible magnetic disks which may be packaged in cassette or cartridge form. More specifically, our invention deals with such a data transfer apparatus having a disk drive motor for imparting rotation to the record medium, and a head transport motor for transporting a transducer head, or a pair of such heads, radially of the record medium for accessing the individual data storage tracks thereon. Our invention is also specifically directed to a method of operation for such a data transfer apparatus.
Generally, in data transfer apparatus of the type in question, the head transport motor is what is known as a stepping motor, having a rotor that rotates in short and essentially uniform angular movements, rather continuously, in response to stepping pulses supplied from an external control. The stepwise rotation of the head transport motor is translated by a suitable rotary to linear converter into the linear stepwise motion of the transducer head or heads for track to track accessing on the record disk.
A problem has existed with this type of head transport motor in data transfer apparatus. Transported to any desired track position on the record disk, the transducer head or heads must be held in that position until the head transport motor rotates in response to the the next series of stepping pulses. The rotor of the head transport motor must of course be retained in a corresponding angular position with respect to the fixed windings of the motor in order to hold the transducer head or heads in the desired track position. Take, for example, a four phase stepping motor of one phase drive. If the third phase windings of this motor have been energized by the last of the preceding series of stepping pulses, the rotor must be held opposite the third phase windings pending the next series of stepping pulses.
A conventional solution to this problem has been to continue, during the absence of stepping pulses, the energization of the head transport motor windings that have been energized by the last of the preceding series of stepping pulses, with a current of either the same magnitude as that during stepping operation or of a smaller magnitude for saving power. We object to the continued energization of the head transport motor as it involves a considerable waste of energy. It is difficult, however, to hold the rotor in the exact angular position required if the motor is energized with a current of the smaller magnitude, and even more difficult if the motor is deenergized. Particularly if the motor is deenergized, the rotor is very likely to be displaced up to approximately 10 to 20 microns from the required angular position, being no longer electromagnetically retained in that position. Such undesired displacement of the rotor, and in consequence of the transducer head or heads with respect to the tracks of the record disk, will also occur because of mechanical vibrations or shocks produced as when the disk drive motor is set into or out of rotation or when the disk is loaded into or unloaded from the apparatus.